This invention relates to xylene isomerization catalyst mixtures based upon supported AMS-1B crystalline, borosilicate molecular sieve catalyst compositions, and particularly, to isomerization of an unextracted, ethylbenzene-containing xylene stream using such mixtures, which process converts ethylbenzene to benzene and ethane primarily by hydrodeethylation and has improved paraffins and naphthenes conversion. More particularly, it relates to catalyst mixtures comprising an AMS-1B crystalline, borosilicate molecular sieve incorporated into an inorganic matrix and silica-supported molybdenum and to processes for using these catalyst mixtures to isomerize an unextracted, ethylbenzene-containing xylene stream to a mixture rich in paraxylene in a process which shows improved paraffins and naphthenes conversion to light hydrocarbons and converts ethylbenzene primarily by hydrodeethylation to benzene and ethane.
Typically, paraxylene is derived from mixtures of C.sub.8 aromatics separated from such raw materials as petroleum naphthas, particularly reformates, usually by isomerization followed by, for example, lower-temperature crystallization of the paraxylene with recycle of the crystallizer liquid phase to the isomerizer. Principal raw materials are catalytically reformed naphthas and petroleum distillates. The fractions from these sources that contain C.sub.8 aromatics vary quite widely in composition but will usually contain 10 to 35 weight percent ethylbenzene and up to about 10 weight percent primarily C.sub.9 paraffins and naphthenes with the remainder being primarily xylenes divided approximately 50 weight percent meta, and 25 percent each of the ortho and para isomers. The primarily C.sub.9 paraffins and naphthenes can be removed substantially by extraction to produce what are termed "extracted" xylene feeds, however, the extraction step adds to processing costs. Feeds that do not have the primarily C.sub.9 paraffins and naphthenes removed by extraction are termed "unextracted" xylene feeds.
The ethylbenzene in a xylene mixture is very difficult to separate from the other components due to similar volatility, and, if it can be converted during isomerization to products more readily separated from the xylenes, buildup of ethylbenzene in the recycle loop is prevented and process economics are greatly improved. The primarily C.sub.9 paraffins and naphthenes present in unextracted feeds unless removed also build up in the recycle loop and are usually extracted prior to isomerization as most commercial isomerization processes do not provide a catalyst which effectively converts them to easily separable-by-distillation products. Thus, it would be valuable to have a catalyst/process for xylene isomerization which would effectively convert both the ethylbenzene and primarily C.sub.9 paraffins and naphthenes to easily separable products without affecting the isomerization efficiency. In addition, the catalyst should minimize xylene loss via hydrogenation and cracking.
Xylene isomerization catalysts can be classified into three types based upon the manner in which they convert ethylbenzene: (1) naphthene pool catalysts, (2) transalkylation catalysts, and (3) hydrodeethylation catalysts.
Naphthene pool catalysts are capable of converting a portion of the ethylbenzene to xylenes via naphthene intermediates. These catalysts contain a strong hydrogenation function, such as platinum, and an acid function, such as chlorided alumina, amorphous silica-alumina, or a molecular sieve. The role of the hydrogenation function in these catalysts is to hydrogenate the C.sub.8 aromatics to establish essentially equilibrium between the C.sub.8 aromatics and the C.sub.8 cyclohexanes. The acid function interconverts ethylcyclohexane and the dimethylcylohexanes via cyclopentane intermediates. These C.sub.8 cycloparaffins form the so-called naphthene pool.
It is necessary to operate naphthene pool catalysts at conditions that allow the formation of a sizable naphthene pool to allow efficient conversion of ethylbenzene to xylenes. Unfortunately, naphthenes can crack on the acid function of the catalyst, and the rate of cracking increases with the size of the naphthene pool. Naphthene cracking leads to high xylene loss, and the by-products produced by naphthene cracking are low-valued paraffins. Thus, naphthene pool catalysts are generally less economic than the transalkylation-type and hydrodee- thylation-type catalysts.
The transalkylation catalysts generally contain a shape selective molecular sieve. A shape selective catalyst is one that prevents some reactions from occurring based on the size of the reactants, products, or intermediates involved. In the case of common transalkylation catalysts, the molecular sieve contains pores that are apparently large enough to allow ethyl transfer to occur via a dealkylation/realkylation mechanism, but small enough to substantially suppress methyl transfer which requires the formation of a bulky biphenylalkane intermediate. The ability of transalkylation catalysts to catalyze ethyl transfer while suppressing methyl transfer allows these catalysts to convert ethylbenzene while minimizing xylene loss via xylene disproportionation.
When ethyl transfer occurs primarily by dealkylation/realkylation, it is possible to intercept and hydrogenate the ethylene intermediate involved with this mechanism of ethyl transfer by adding a hydrogenation function to the catalyst. The primary route for converting ethylbenzene then becomes hydrodeethylation, which is the conversion of ethylbenzene to benzene and ethane. It is desirable to selectively hydrogenate the ethylene intermediate without hydrogenating aromatics (without establishing a naphthene pool) to prevent the cracking of the naphthenes that occurs over the acid function of the catalyst. Commercial hydrodeethylation catalysts selectively hydrogenate ethylene without substantial hydrogenation of aromatics at reported commercial conditions.
In order to form a hydrodeethylation catalyst, it is essential to use an acidic component that behaves as a shape selective catalyst, i.e., one that suppresses the formation of the bulky biphenylalkane intermediate required for transmethylation, because transethylation can occur via a similar intermediate. For catalysts with pores large enough to allow the formation of these biphenylalkane intermediates, transethylation appears to occur primarily via these intermediates. In this case, ethylene is not an intermediate for transethylation, and the addition of a hydrogenation component cannot produce a hydrodeethylation catalyst
Molecular sieves such as the AMS-1B crystalline, borosilicate molecular sieves have shown great utility in the isomerization of xylenes to make primarily paraxylene Such sieves when supported on an oxide carrier like alumina effectively produce equilibrium amounts of paraxylene and dispose of ethylbenzene largely by transalkylation without serious loss of xylenes. However, such sieves are not very effective in removing paraffins and naphthenes during the isomerization of xylenes and they are generally used with extracted feeds.
Periodic Group VIb elements including molybdenum have shown utility in the past for various hydrocarbon conversions including hydrogenation. In particular, in U.S. Pat. Nos. 4,420,467; 4,532,226; and 4,655,255, molybdenum is said to be incorporated into or on a molecular sieve framework, which sieve is useful for hydrocarbon conversions including isomerization. In U.S. Pat. No. 4,202,996, hydrocarbon isomerization is carried out over a catalytic composite having a nickel component, a molybdenum component, a platinum component in combination with a zeolitic carrier. In other work, the activity of supported molybdenum compounds useful for hydrogenation/dehydrogenation has been found to depend upon the oxidation state of molybdenum with the lower molybdenum oxidation states being more effective.
Now it has been found that by adding molybdenum on silica to an alumina-supported HAMS-1B crystalline, borosilicate molecular sieve catalyst composition, a catalyst mixture is formed which, when used for xylene isomerization of unextracted xylene streams, removes ethylbenzene primarily by the hydrodeethylation mechanism to benzene and ethane and can substantially increase the removal of paraffins and naphthenes by cracking to light hydrocarbons. These results are obtained, moreover, without otherwise substantially affecting the isomerization effectiveness of the supported molecular sieve catalyst composition. Unexpectedly, other common molybdenum supports such as alumina do not produce a supported molybdenum which is as effective in removing paraffins and naphthenes when made into a catalyst mixture with the borosilicate sieve.